This invention relates to a particle measuring and counting system and, more particularly, to a particle measuring and counting system with an improved particle sensing cell having special applicability for use in a vacuum line and semiconductor processing equipment.
Particle measuring systems have been developed wherein the particles to be measured are entrained in a fluid stream which is passed through a light beam, typically a laser light beam. Particles passing through the beam will scatter light which is collected and focused on a photodetector or photodetectors resulting in electrical pulses being generated. The intensity of the scattered light and, accordingly, the amplitude of the pulses generated by the photo-detectors provide an indication of the particle sizes.
In many systems, it is important to detect every particle in the entrained fluid stream. This need requires that the light beam pass through the entire cross sectional area of the fluid stream entraining the particles. When the fluid stream is a liquid stream, the conduit carrying the stream through the light beam is typically narrowed down to a small cross sectional area so that a high intensity beam can be caused to pass through the entire cross sectional area of the fluid stream. When the fluid stream entraining the particles is a gas stream of substantial density, the gas stream is shaped by a nozzle into the form of the sheet and a laser beam is passed through the sheet-shaped stream along its long dimension as described, for example, in U.S. Pat. No. 4,746,215 to Kenneth P. Gross. However, when the particles are in vacuum, as in semiconductor processing equipment, nozzles cannot readily be used to shape the stream into a sheet-like form. In addition, in order for the vacuum line to efficiently transmit a vacuum to the work area of the semiconductor processing, it is necessary for the vacuum line to have a substantial cross sectional area. Thus, the prior art particle detecting systems involving narrow conduits or nozzles for shaping the fluid stream have limitations which make them not fully satisfactory in detecting particles in a vacuum line such as is employed in semiconductor processing equipment.